When you first come across the term spongy bone, the first thought that enters your mind may well be the term describes a bone that has become weakened by disease, and is riddled with holes and canals. We often think of our bones as being like the hard plastic bones we purchase as a chew-toy for a puppy, a bone that is hard and solid throughout.

Our Bones Are Compact Or Spongy, But Never Completely Solid

Our bones are not hard and solid throughout, though most are quite solid on the surface. Some bones are mostly solid, and some are only partially solid. The correct term for what we might call a solid bone is a “compact bone”. Bone is living tissue, and therefore it can never be completely solid. Even though compact bone may appear to be solid, there are blood supplying canals running through it. Spongy bones are characterized by more and larger canals per unit volume of bone. Our bones are made up of compact parts and spongy parts, the percentage of each part in a given bone being dependent upon the function of the bone.

Many longer bones, such as those in our legs, are comprised mostly of compact bone, as they need to be strong to support the body in performing its daily activities. The ends of these long bones, such as at a joint, may primarily consist of spongy bone. Short bones, those which are roughly as wide as they are long, do not need to have the strength of the long bones. In home building vernacular, the long bones would be equivalent to load-bearing beams, which need to be strong and heavy. Predominantly spongy bones, such as those found in the wrist, are lighter, more porous, and slightly more flexible. They are however covered with a thin layer of protective compact bone.

By Design – Not By Disease

Spongy bone, also called cancellous bone, may be weaker than compact bone, but that is due to design rather than disease. Because it is lighter, and because of its larger surface area per unit weight, spongy bone is the ideal bone for the vertebrae in the spinal column. Each vertebra has a protective coating of compact bone, but each is made up mostly of spongy or cancellous bone. When our entire skeletal system is taken into account, about 80% of it is made up of compact bone, and the other 20% of cancellous bone. Bone marrow, where blood cells are produced, is located in spongy or cancellous bone.

Bone Diseases

Bones are susceptible to disease, just as any other tissue in the body, but are generally less susceptible to disease than are the soft tissues. Nevertheless, bone diseases do occur, and when they do the spongy or cancellous bone is most often affected. The most common disease affecting spongy bone is osteoporosis, where calcium is displaced from the cells, keeping the bone from growing, or repairing itself. Roughly half the population over 50 years of age has this disease to some degree. It is most common in post-menopausal women. If left unchecked, or allowed to run its course, osteoporosis can make bones brittle, and easy to break. Osteoporosis is often brought on by a hormone imbalance, which is one reason that hip fractures are so common among older women. A somewhat similar disorder is osteomalacia. Osteomalacia doesn't remove calcium from bones, as osteoporosis does. Rather, it tends to keep calcium from being made available to the bones in the first place. Osteomalacia can usually be traced to a nutritional deficiency of some kind, and therefore is more easily prevented.

Yet another problem that can affect the soft bones is hyperparathyroidism. The parathyroid gland functions to regulate the amount of vitamin D and calcium in the blood. If it detects a deficiency, the gland will borrow calcium from the soft bones. It should be noted at this point that one of the functions of the bones in our skeletal system is to act as a depository for certain mineral salts. So it is quite normal for minerals to be borrowed from the bones. If the parathyroid gland becomes dysfunctional, it may overreact, and continually remove calcium from the bones, even when there is no need to do so. Eventually the bones will weaken.

The above diseases are for the most part treatable. In some instances, medication may be needed; in other instances a change in dietary habits, coupled with exercise, may be all that's required. It's especially important for post-menopausal women to take the necessary steps to protect against hormonal imbalance, especially a deficiency in estrogen. Preventive treatment often includes a combination of medication, supplements, dietary changes, and exercise.

In summary, spongy or cancellous bones are not diseased bones; they are a natural and important part of our skeletal system. They are weaker and lighter than compact bone, but are specifically designed to be so. As noted above, they can become diseased, but when appropriate preventive measures are taken, that should seldom happen.